1. Field of the Invention
Generally, the present invention relates to communication networks. More specifically, the invention relates to an apparatus and method for determining the physical location of a mobile telephone within a cellular communication network.
2. Description of the Related Art
Recent developments in global positioning satellite networks (GPS) and terrestrial mobile communications make it desirable to integrate GPS functionality into a mobile communication device, such as a mobile telephone, more generally referred to herein as a remote station, in order to support various position location functions and features. A wireless link exists between a mobile communication device within a terrestrial mobile communications system and a base station within the communications system. A base station is typically a stationary communication device that receives wireless communications from, and transmits wireless communications to, a wireless mobile communication device. The base station also communicates with communication networks to complete the connection between the mobile communication device and an end-point device, such as another mobile communication device, a conventional telephone, a computer or any other such device. This wireless link may be used to communicate position location information between the mobile communication device and the base station in order to improve the performance of the GPS receiver within the mobile communication device. In particular, certain functions that must be performed in order to locate the position of a mobile communication device in accordance with a GPS system can be performed by the base station, rather than by the communication device. By xe2x80x9coff-loadingxe2x80x9d some of the functions to the base station, the complexity of the communication device can be reduced. Furthermore, since the base station is stationary, the location of the base station can be used to assist in locating the position of the communication device.
Many services, such as CDMA Tiered Services (described in industry standard TR45.5.2.3/98.10.xx.xx, CDMA Tiered Services Stage 2 Description, Version 1.1, published by the Telecommunication Industry Association/Electronics Industry Association (TIA/EIA)), require a wireless telephone to be capable of determining its location while in an idle state. The location must then be displayed to the user. In the idle state, the wireless telephone monitors transmissions from a base station over a control channel broadcast by the base station. For example, in an industry standard IS-95 CDMA system (as defined by industry standard IS-95, published by the TIA/EIA), the base station transmits a paging channel. Each of the telephones capable of receiving signals from a particular base station will monitor information broadcast on the control channel to determine whether incoming calls or other data are intended for that telephone.
A GPS receiver typically measures the range to at least four GPS satellites. If the locations of the satellites and the ranges from the phone to the satellite are known at the time the measurement is made, then the location of the phone can be computed. Since GPS satellites orbit around the Earth, the relative position of the GPS satellites with respect to the earth changes with time. The location of a GPS satellite can be determined by having a description of the orbit of the satellite along with the time when the satellite position is to be computed. The orbits of GPS satellites are typically modeled as a modified elliptical orbit with correction terms to account for various perturbations.
Certain methods for computing the location of a device require measuring the ranges to the satellites at the wireless phone, and then transmitting these ranges to a server connected to the base station. The base station uses these ranges, along with the locations of the satellites at the time the range measurements were made, to compute the location of the phone. This computed location may be displayed to the user or sent to any other entity that needs the location. This method is suitable for a phone that has a dedicated traffic channel. However, the method is not suitable for phones in the idle state, because the phone lacks a dedicated traffic channel over which to send the information to the base station during the idle state.
In the absence of a dedicated traffic channel over which to communicate with the base station, the phone may use a shared access channel to send information to the base station. However, transmitting measured ranges to the base station over a shared access channel, a channel commonly used to establish a call to or from the phone, can have a significant impact on the capacity of the shared access channel and on the life of the battery that powers the phone. Hence, it is not practical to transmit measured ranges to the base station. In this case, the phone must compute its own location. To do so, the phone must know the locations of the GPS satellites, and the errors in the GPS satellite clock, because an accurate GPS satellite clock is required to determine the range measurements accurately. This information is transmitted to the phone over the control channel. However, even transmitting this information to the phone creates a significant burden on the control channel.
Under conventional conditions, the control channel has to carry large amounts of access information. The control channel has a very limited capacity to carry messages. Hence, it is not possible to convey extensive GPS information over the control channel. Furthermore, the information must be transmitted in a form that allows the information to be used for a relatively long time after it has been received.
These problems and deficiencies are recognized and solved by the present invention in the manner described below. The invention is compatible with CDMA modulation systems and the techniques used in telecommunication systems servicing large numbers of system users. A more robust discussion of CDMA systems and techniques in this type of multiple access communication systems may be found in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Further, the invention ideally would be adaptable to other modulation systems and techniques used in other communication systems, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AM) schemes.
Broadly, the present invention relates to locating a remote station in a communication network employing wireless links. More specifically, the invention uses an inverted GPS approach to locate remote stations that may not have a sense of network time. The invention also enables remote stations that do not know true GPS time to determine their own location.
In one embodiment, the present invention provides a method to solve this problem by time stamping the approximate time a satellite range measurement is made. This timestamp is assigned at either the remote station or elsewhere in the network. The difference between true GPS network time and this measurement time is treated as a variable xe2x80x9cerrorxe2x80x9d in measurement, the length of which is unknown. However, the method of the present invention allows this variable to be determined and used to find the physical location of the remote station.
Of course, the time stamp assigned to the measurements may have a significant error: a few seconds or more. This error will result in an error in the computed location of the remote station. By treating the error in measurement time stamp as an unknown, and solving for this unknown, an accurate location for the remote station can be computed in spite of the erroneous time stamp. This requires that one additional satellite measurement, over the four satellite measurements used for traditional methods, be made.
In another embodiment, the invention provides an article of manufacture containing digital information executable by a digital signal processing device and used to determine the location of the remote station described above. In another embodiment, the invention yields an apparatus used to determine the location of the remote station. The apparatus may comprise a remote station and at least five satellites that include transceivers used, in part, to transmit signals to the remote station. The apparatus may also include at least one base station that has, among other things, a transceiver used to communicate signals to the remote station. These signals include a timing signal used to establish a reference network time. Obviously, to receive signals, the remote station also includes a transceiver communicatively coupled to the base station and satellites, adapted to receive the satellite signals. The apparatus will also include a digital processing apparatus, such as a microprocessor, that is communicatively coupled to the network or one of its component parts.
The invention provides its users with numerous advantages. One advantage is that it speeds up the process of locating a remote station. Another advantage is that the location of the remote station may be determined even if the remote station does not have an estimate of network time. The invention also provides a number of other advantages and benefits that should become even more apparent after reviewing the following detailed descriptions of the invention.